Prostate cancer is one of the most common cancers and its incidence is increasing. It is desirable to be able to diagnose prostate cancer at an early stage when the disease is potentially curable. A number of prostate diseases, including prostate cancer, lead to increased blood levels of prostate-derived proteins such as prostate specific antigen (PSA) and these proteins serve as diagnostic markers for these diseases. Blood levels of PSA, for example, are increased when normal prostatic structure is disrupted by benign or malignant tumours or by inflammation.
Testing of PSA has increased rapidly among asymptomatic men in North America, and recommendations favouring such screening have been issued by the American Cancer Society and the American Urological Association. About 70% of patients with cancers identified by digital rectal examination, transrectal ultrasound, and needle biopsy have PSA concentrations above the cut-off values generally used. About 70-80% of abnormal PSA values in asymptomatic men, however, are false positives; only 1 in 4 men with a PSA above 4 .mu.g/L will have prostate cancer (Catalona et al., (1991), New England Journal of Medicine, 324, 1156-1161; Chadwick et al., (1991), Lancet, 338, 613-616). The diagnostic potential of PSA measurements in serum is limited by the increase of PSA concentrations in many subjects with benign prostatic hyperplasia (BPH).
PSA has been shown to exist in serum in complexes with antichymotrypsin (ACT), .alpha..sub.2 macroglobulin (AMG), protein C inhibitor and pregnancy zone protein.
When the ratio of PSA-ACT complex: total PSA was determined, 38-60% of false positives in the range 2.5-25 .mu.g/L were eliminated, giving improved diagnostic accuracy without loss of sensitivity.
Measurement of serum PSA is not, however, totally satisfactory for diagnosis of prostate disease and the use of multiple markers has been suggested.
PSA (also known as .alpha.-microseminoprotein) and PSP94 (prostate secretory protein of 94 amino acids, also known as .beta.-microseminoprotein) are the two most abundant secretory proteins from prostate and PSP94 has been examined by a number of groups as a possible blood marker for prostate disease, using radioimmunoassay, enzyme immunoassay, two site immunoradiometric assay and enzyme linked in immunosorbent assay.
These studies have produced conflicting results on the efficacy of serum PSP94 as a marker for prostate cancer (van der Kammer et al., (1993), Urol. Res., 21, 227-233).